From the Equipment chapter of the Basic Rules.
Thrown. If a weapon has the thrown property, you can throw the weapon to make a ranged attack.
The attack is a ranged attack using a weapon, which is a "ranged weapon attack"¹, so the Monk feature applies.
If someone throws an item that is not a weapon with the thrown tag then it is an improvised weapon. You are still making an attack roll, so it is a ranged weapon attack. (Hat-tip @Rubiksmoose.)
An object that bears no resemblance to a weapon deals 1d4 damage (the DM assigns a damage type appropriate to the object). If a character uses a ranged weapon to make a melee attack, or throws a melee weapon that does not have the thrown property, it also deals 1d4 damage. An improvised thrown weapon has a normal range of 20 feet and a long range of 60 feet.
¹ The Equipment chapter does not use the phrase "ranged weapon attack" but the Combat chapter does, though only twice (one of which is in the Underwater Combat section).
Ability Modifier. The ability modifier used for a melee weapon attack is Strength, and the ability modifier used for a ranged weapon attack is Dexterity. Weapons that have the finesse or thrown property break this rule.